Miscarriages and TTCAL are heartbreaking and it sounds like you've had a lot of other heartbreak and trauma to deal with as well. I'm concerned that not only have you had the loss, but you then had a fiancé who beat you while you were TTC and then you immediately started to TTC with your current boyfriend. I mean this in the most loving, gentlest way possible, but I'm concerned. To go from TTC with an abusive fiancé to then immediately TTC with a new partner seems a little unhealthy. Obviously I only know the few facts you laid out in your post, but from what you said, I would suggest that you might want to consider meeting with a therapist to work through your prior loss, as well as the abuse and what's motivating you to TTC so soon with multiple men. Your new partner might be absolutely amazing and supportive, but it would have been hard to have known that when you first got together and immediately started TTC. It sounds to me like having the baby is more important to you than who you have the baby WITH. But the father will be in your life for the rest of your life, even if you don't stay together, and having a child puts an incredible strain on a relationship. It just does. And who the father is will have a huge impact (obviously) on your baby's life, so it is not a decision that should be made lightly. Having a baby just for the sake of having a baby is not fair to the child.
It also sounds like you are also around a lot of people who get pregnant young and might not be the most sophisticated when it comes to pregnancy and childbirth (i.e. only learning at 21 weeks that she's pregnant). Everyone makes their own choices, but honestly, there is nothing glamorous about having a baby at 17. That makes my heart break, more than anything else. At 17 there are so many experiences your brother should still be having that he will not be able to have (at least for a long time) once the baby comes.
A baby is amazing, but it changes your life in so many ways. My husband and I had been married for four years when we started TTC. Our rainbow baby is 5 months now and our six year anniversary is next month. We have both been to college (and I had been to grad school). We've partied and traveled (alone and together) and bought two houses together before having our baby. But despite all of that, there is still so much we can't do now with her and things are harder. Our choices are limited and it's much harder to connect as a couple. Where we used to just put on our shoes and walk out the door to go to the store or go on a date, we now need to pack bottles and a stroller and a diaper bag (and forget about it at all if she's napping). If we want to go on a date, it needs to be planned several days in advance, so we can arrange for a babysitter. And there's no partying, since one or both of us needs to be alert enough to wake up and care for her in the middle of the night. So yes, having a baby is amazing, but it completely changes your life and limits your options and it's not anything I would wish on a 17 year old.
With all of that said, while I would still urge you to consider talking to a therapist to discuss the loss, the abuse, and what's motivating you to TTC right away with multiple men, if this is the path you truly want, after 2 years of TTC (since you started in March 2012, I assume), you should be able to meet with a fertility specialist to have tests done and rule out potential fertility problems for both you and your current partner. Sometimes there are simple things preventing conception and healthy pregnancies that are easily fixable once they're diagnosed (like low progesterone or blood clotting problems). Most RE's will meet with patients after 1 year of TTC, so you should have no problem getting an appointment.
Infertility and loss are common problems, so you're not alone in the world at all. Most (if not all) of the women on this board have dealt with these issues as well.
